Godot: Editing a project in 3.1 makes it impossible to open in 3.0.x

[MattUV]

Godot version: v3.1.alpha.02490db and 3.0.5.stable

OS/device including versionWindows 10 : Windows 10

Issue description:
If you edit a project in Godot 3.1, you cannot open it anymore in Godot 3.0. It throws this error:

ERROR: _load_settings_text: Condition ' config_version > 3 ' is true. returned: ERR_FILE_CANT_OPEN
   At: core/project_settings.cpp:495
ERROR: _load_settings_text_or_binary: Couldn't load file 'C:/Users/matth/Documents/Godot/testBug/project.godot', error code 12.
   At: core/project_settings.cpp:519

Steps to reproduce:

• Create a project in v 3.1 and try to open it in Godot 3.0.
• OR : Create a project in 3.0, edit it in 3.1 and try to open it again in 3.0

Minimal reproduction project:
This project was made in 3.1, I cannot open it again in 3.0
testBug.zip

[AlexHolly]

It is not a bug
Open project.godot and edit config_version=4 to config_version=3

[MattUV]

Thank you very much!
Also, this post was a duplicate of #18758. I am closing it.